With the introduction of new compression algorithms into broadcast systems, video signals which would previously have been compressed and decompressed using MPEG-2 encoding now often undergo several compression/decompression stages using MPEG-2 encoding as well as more advanced algorithms such as H.264 encoding. Whereas in the past, successive MPEG-2 to MPEG-2 encoding/decoding was the predominant mode of concatenation, today's systems often require MPEG-2 to H.264 as well as H.264 back to MPEG-2 concatenation. Once H.264 compression is used for distribution, as well as final transmission to the home, H.264 to H.264 concatenation will be the predominant mode of concatenated compression.
MPEG-2 as well as H.264 algorithms use three types of picture coding: intra coded pictures (I pictures) which are coded without taking predictions from other pictures; forward predicted pictures (P pictures) which are predicted from previously coded I or P pictures and bi-directionally predicted pictures (B pictures) which take predictions from past and future I or P pictures. In H.264, B pictures can also take predictions from other B pictures.
Since the amount of compression varies from one picture type to another, that is B pictures are usually more heavily compressed than P pictures which, in turn are more heavily compressed than I pictures, picture type alignment between successive compression/decompression stages is the most important factor influencing the performance of concatenated compression systems (A. M. Bock, ‘Near Loss-Less MPEG Concatenation without Helper Signals’, IBC, Amsterdam, September 2001). In particular, if B pictures are used in the downstream encoder these should be aligned with pictures which have previously also been coded as B pictures. This requires that the two encoders not only use a same number of B pictures between I and P pictures but also that the downstream encoder is locked to a coding pattern of the upstream encoder.
Picture type alignment between MPEG-2 compression/decompression systems can be achieved using a method such as that disclosed in GB 2347039, which discloses a method of finding an I picture of a previously MPEG-2 compressed video signal, preferably using a DCT (Discrete Cosine Transform) filter for the detection. Thus, in particular, GB 2347039 discloses using energy levels to determine a picture type and using DCT parameters to determine quantisation level. Since H.264 compression uses two sizes of pixel blocks to form the transform (4×4 and 8×8), both different from the DCT used in MPEG-2, the algorithm described in GB 2347039 is not suitable for the detection of H.264 intra coded pictures. A more general approach to the detection of previously coded picture types is required.